Mid-Infrared Spectral Diagnostics of Nuclear and Extranuclear Regions in Nearby Galaxies

Dale, D. A.; Smith, J. D. T.; Armus, L.; Buckalew, B. A.; Helou, G.; Kennicutt, R. C., Jr.; Moustakas, J.; Roussel, H.; Sheth, K.; Bendo, G. J.; Calzetti, D.; Draine, B. T.; Engelbracht, C. W.; Gordon, K. D.; Hollenbach, D. J.; Jarrett, T. H.; Kewley, L. J.; Leitherer, C.; Li, A.; Malhotra, S.; Murphy, E. J.; & Walter, F.
2006, The Astrophysical Journal, 646, 161

Mid-infrared diagnostics are presented for a large portion of the Spitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archival data from ISO and Spitzer. The SINGS data set includes low- and high-resolution spectral maps and broadband imaging in the infrared for over 160 nuclear and extranuclear regions within 75 nearby galaxies spanning a wide range of morphologies, metallicities, luminosities, and star formation rates. Our main result is that these mid-infrared diagnostics effectively constrain a target's dominant power source. The combination of a high-ionization line index and PAH strength serves as an efficient discriminant between AGNs and star-forming nuclei, confirming progress made with ISO spectroscopy on starbursting and ultraluminous infrared galaxies. The sensitivity of Spitzer allows us to probe fainter nuclear and star-forming regions within galaxy disks. We find that both star-forming nuclei and extranuclear regions stand apart from nuclei that are powered by Seyfert or LINER activity. In fact, we identify areas within four diagnostic diagrams containing >90% Seyfert/LINER nuclei or >90% H II regions/H II nuclei. We also find that, compared to starbursting nuclei, extranuclear regions typically separate even further from AGNs, especially for low-metallicity extranuclear environments. In addition, instead of the traditional mid-infrared approach to differentiating between AGNs and star-forming sources that utilizes relatively weak high-ionization lines, we show that strong low-ionization cooling lines of X-ray-dominated regions like [Si II] 34.82 μm can alternatively be used as excellent discriminants. Finally, the typical target in this sample shows relatively modest interstellar electron density (~400 cm-3) and obscuration (AV~1.0 mag for a foreground screen), consistent with a lack of dense clumps of highly obscured gas and dust residing in the emitting regions.


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